The characteristics of western boundary currents are better known than those along the eastern edges of oceans. In the 1970s, however, close attention has been given to eastern boundary currents. The variability of the El Nino phenomenon off Peru, and coastal upwelling off Oregon, Peru/Chile, and Northwest Africa, have been studied by multi-national teams. Since 1975, U.S. Navy personnel and scientists sponsored by the Office of Naval Research have examined satellite imagery of the waters off California and Western Europe as a first step in real-time satellite oceanography. From these studies, we know now that eastern boundary currents have identifiable fronts (such as the Huelva front) and eddies (like the Socal eddy chain) that vary seasonally and inter-annually in intensity and scale. The fronts have lengths of tens of kilometres and depths of hundreds of metres; the eddies have diameters of 50 to 200 km and depths greater than 500 m. Both the eddies and fronts have surface temperature gradients of 1.5 C to 5 C. Deeper, the temperature gradients tend to be sharper than those at the surface. The data are useful therefore in solving tactical ASW problems.